The first portion of the background relates to passenger locomotive HEP and prime exhaust merging into a single main SCR system for lower emissions. Passenger locomotives are distinct from freight locomotives in that they have to provide not only tractive power from the prime engine to propel the locomotive, but also have to provide hotel power to provide lights and climate control for the passenger cars. Providing this hotel power is typically done with a second diesel engine and generator called a head-end power (HEP) generator.
In 2010 Metrolink tested a retrofit selective catalytic reduction (SCR) system on its F59PH locomotive SCAX 865. This system only treated the exhaust gasses from the 3000 hp main engine used for propulsion, the 250 kW HEP generator was not modified to lower its emissions. This demonstration program was funded by several agencies including the South Coast AQMD and CARB. It was designed, built and installed by Engine, Fuel and Emissions Engineering, Inc out of Rancho Cordova, Calif.
Two particular challenges came up in this testing program. The first challenge was low exhaust temperatures at throttle positions below notch 3, and the second was the challenge of getting the UREA fluid to mix well with the exhaust gasses.
SCR systems from this same company will typically have NOx reduction efficiencies above 95%. Two conditions are required to meet this high efficiency level. The catalyst must be above a minimum operating temperature and the urea fluid must be vaporized and well mixed with the exhaust gasses. The base engine in this test program was tested at 9 g/(hp-hr) of NOx and the SCR system was able to reduce that to 2.6 g/(hp-hr), an overall emissions reduction efficiency of 71%. In order to achieve Tier 4 emissions levels of 1.3 g/(hp-hr) it would need an overall efficiency of 85%, which is well within the capabilities of the technology. Typical SCR installations would have a long mixing pipe that the exhaust gas and urea mixture would flow through before reaching the SCR substrates where the NOx reduction happens. A typical mixing duct has a length equal to 10 times its diameter.
Because of the tight packaging constraints of the locomotive, the SCR system was mounted immediately above the engine turbo charger outlet. At high loads and temperatures, the efficiency of this system was slightly above 80% instead of over 95% typical for an SCR system at operating temperature. These low efficiency numbers at higher operating temperatures illustrate the severity of the urea mixing issue and how it limited the overall efficiency of the SCR system.
In addition to poor urea mixing, low exhaust temperatures started to significantly affect the SCR system emissions reduction efficiency below throttle notch 4; at notch 3 the efficiency was reduced to 73% and by notch 2 it was down to 28%. At notch 1 and at idle the SCR system was inactive due to the SCR catalyst temperature being below the temperature required for thermal dissociation of the urea. The system was programmed to shut off urea injection under these conditions. It is these very low efficiencies at lower temperatures that brought the overall system efficiency down to 71% when at higher loads it was typically over 80%.
What is needed is an economical retrofit system for existing passenger locomotives that will solve these two system shortcomings so that the system will reduce NOx emissions below future EPA Tier 4 levels. It would also benefit the end user if this system could also help reduce the NOx emissions of the HEP generator.
The second portion of the background relates to combined cooling systems for PM emissions compliance and thermal efficiency. In the railroad industry, it has been a technique to reduce idling for many years by adding what is called an auxiliary engine to a locomotive to reduce the amount of time the main engine is idling. As far back as 1984, the Locomotive Cyclopedia had an advertisement for a system by Microphor. This reduction in main engine idle time saves wear and tear on the main engine, reduces emissions and saves fuel. When these auxiliary engines are liquid cooled they will transfer heat to the main engine coolant by transferring heat from both the auxiliary engine exhaust and the auxiliary engine coolant. These engines are typically very small, under 25 kW and these have typically been installed on freight locomotives.
As previously stated, passenger locomotives are distinct from freight locomotives in that they have to provide not only tractive power from the prime engine to propel the locomotive, but also have to provide hotel power to provide lights and climate control for the passenger cars. Providing this hotel power is typically done with a second diesel engine and generator called a HEP generator. Unlike auxiliary engines used for engine heating that are under 25 kW in power, HEP generators used in passenger locomotives are typically 250 kW or more. Currently Metrolink is specifying 600 kW HEP generators for new locomotives as they expect to be pulling longer trains with 10 passenger cars in the future.
Locomotives emissions requirements under EPA guidelines are different for HEP generators and auxiliary engines. A passenger locomotive engine that only provides hotel power does not fall under the locomotive emissions rules, but falls under off-road rules. If this engine performs any function beyond providing hotel power it will lose this exception and its emissions will somehow have to be combined with the prime engine emissions when emissions testing the locomotive engine. To this day, there has not been a passenger locomotive prime engine and HEP engine certified together; but the EPA regulations clearly acknowledge that a locomotive can be certified under an alternative duty cycle that would be developed for this specific application.
HEP generators are typically high speed diesel engines as used in class 8 trucks and capable of NOx emissions 85% below the locomotive Tier 4 standard and PM emissions 66% below the locomotive Tier 4 standard with the use of both an SCR system and a particulate filter system.
Engine, Fuel and Emissions Engineering Inc. has demonstrated a Compact SCR system for the EMD main engine that has the potential to reduce NOx emissions below Tier 4 levels with some further development. This Compact SCR system also has a section that acts as an oxidation catalyst that reduces particulate matter (PM). It is likely that this system with modern low oil consumption piston rings and liners will achieve PM emissions below Tier 3 levels, but not below Tier 4. Because of the scavenging nature of uniflow 2 stroke engines, they are not tolerant of significant increases in exhaust back pressure and it is impractical to put a particulate filter on them. This is a major reason why the 2 stroke truck engines were phased out by Detroit Diesel when the EPA starting imposing emissions limits on diesel truck engines.
It could be possible to economically retrofit older passenger locomotives to meet Tier 4 PM emissions levels if there was a practical way to combine the very low PM emissions level of the 4 stroke diesel particulate filter (DPF) equipped HEP engine with the slightly higher than Tier 4 PM emissions of an updated EMD 2 stroke prime engine.
The third portion of the background relates to combined high temperature and low temperature coolant loops with a thermal reservoir. Locomotives consume almost 5% of their fuel when powering the engine cooling fans. This energy could be saved by using ram air cooling, but that solution is not practical in rail applications for several reasons. First, freight trains can be moving slowly at high power for extended periods of time as when climbing a hill or starting from a stop with a very long train. Second, locomotives typically have to travel in either direction so the air ducting system would have to be bidirectional.
Passenger locomotives in commuter rail service do not have the issue of low-speed high-load operation for extended times. The passenger application is actually a higher speed application with frequent stops. With a high average speed the passenger locomotive would appear to be a good candidate for ram air cooling. But there is one issue. The higher speed operation of the passenger train where adequate ram air cooling is available does not occur when the locomotive is generating the most waste heat. The time when the most engine cooling is needed is when leaving the station after a stop and accelerating up to speed. This opposite timing of high cruising speeds and high engine loading makes using ram air cooling impractical even for this high average speed locomotive application.
Waste heat recovery is another technology that would be a good fit for passenger locomotives as they consume a lot of fuel and thus have a lot of waste energy to recover. They have steel wheels so the excess weight of the additional waste heat recovery equipment is not as much a detriment as it would be on a rubber tired on-road truck.
With the coming transition to natural gas and hybridization that will require a tender car, even the space is now available to install the needed ducting for bi-directional ram air cooling which could cool the prime engine and the HEP generator. But the opposite timing of high engine loading versus high speed makes it impractical.
What the locomotive system needs is a simple and reliable way to decouple the timing of when waste heat is created from when it can be rejected into the atmosphere.